1. Field of the Invention
The present invention relates to a PC strand manufactured by coating a core wire and surrounding wires of a PC strand used as tensioning member or stay cable for post-tensioning or pre-tensioning in prestressed concrete used for structures such as architectural constructions and civil engineering structures, or of a PC strands used as stay member or stay cable for marine structures and cable-stayed bridges susceptible to salt corrosion with a plated layer and a synthetic resin coating by a double rustproof processing treatment.
2. Prior Art
In general, a PC strand has a structure having plural surrounding wires twisted around a core wire. The reason for using such a structure is to impart flexibility to the PC strand, and to form helical grooves with the twisted surrounding wires and thus provide a sufficient shear resistance for wires embedded in concrete. Accordingly, there is a need for a treatment method for the PC strand applied with the rustproof processing that does not interfere with these characteristics. In actuality, several PC strands applied with the rustproof processing treatment and rustproof processing treatment methods are known.
As a first known prior art, there is a corrosion-resistant composite member (WO92/08551), which is a corrosion-resistant member having an enhanced resistance with respect to fatigue breakdown, including strands formed of high-strength steel wires, formed with a substantially impermeable, continuous and firm adherent coating of epoxy-based resin on an outer surface of the strand, and filled with the epoxy resin in internal gaps between adjacent steel wires abutting with each other. Accordingly, bending rigidity of the strand is increased, relative movement between the steel wires of the strand is reduced, and a resistance against breakdown due to bending fatigue or chafing fatigue is enhanced. Consequently, the coating and the filling are kept adhered integrally with the strand and its steel wires when being subjected to winding or bending, and when tensed and expanded.
The corrosion-resistant composite member is exposed to the cloud of epoxy-based resin powder charged with static electricity containing air in a temporarily opened state, whereby the bear core wire and surrounding wires are individually coated and the coating material works as a filling material or an impregnating material for gaps or voids when the strand is closed to its original shape immediately thereafter and hence is impregnated in and coated completely on the strand, thereby enhancing the corrosion resistance and, simultaneously, resisting the relative movement of the wires, and increasing bending rigidity which reduces the chafing fatigue and reduces the bending fatigue.
As a second known prior art, there is a method of forming and processing rustproof coatings on PC strand (U.S. Pat. No. 5,362,326A) including temporarily untwisting the PC strand in sequence, maintaining a spread state by the spread maintaining units, adjusting an excess part of the core wire, forming synthetic resin powder coating adherent films on the entire outer peripheral surfaces of the core wire and the surrounding wires of the untwisted portions respectively, heating and welding the adherent films to form coatings on the entire outer peripheral surfaces of the core wire and the surrounding wires respectively, cooling the coatings, and re-twisting the core wire and the surrounding wires.
The PC strand formed in this manner is not subjected to impairment of the characteristics required as the PC strand such as flexibility and shear resistance with respect to concrete because the coatings are formed individually on the respective core wire and surrounding wires over the entire outer peripheral surfaces thereof and, in addition, the rustproof function is sufficient. Therefore, this rustproof method is evaluated to be an ultimate rustproof method for the PC strand.
As the thickness of the coat of this type, in order to satisfy corrosion-resistant performances and dynamic performances (shock resistance, bending property, or adhesive property for concrete), a thickness of 200±50 μm is reported to be suitable for the coat formed of a powder-type epoxy resin according to many results of study, and a range of approximately 170±50 μm is reported to be preferable according to the result of experiment conducted by FHWA (Federal Highway Administration) of the United States of America.
As a third known prior art, there is a method of forming double coatings on a PC strand including untwisting surrounding wires of the PC strand temporarily from the core wire in sequence, and in the untwisted state, forming a rustproof coating on the entire outer peripheral surfaces of the core wire and the surrounding wires respectively, twisting the surrounding wires on the core wire again while integrating and absorbing an excess part of the core wire generated by an increase in diameter, then further forming a coating thereon, which is a method of forming double coatings by forming additionally a thick coating on the outer peripheral surface of the PC strand of the first prior art in a case where there is a risk of occurrence of damage of the rustproof coating used in a special structure and a film thickness of 250 μm or larger which is a maximum thickness of coating which can be stably held, is required (JPA—1999200267).
Furthermore, as a fourth known prior art, there is a method of forming a rustproof coating including forming a PC strand after having applied a wire drawing treatment to plated wires, untwisting the PC strand to apply a blast treatment on a core wire and surrounding wires, forming resin coatings on the outer peripheral surfaces of the core wire and the surrounding wires applied with the blast treatment, and twisting the core wire and the surrounding wires again after having cooled the resin coatings (JPA—2004263320).
With this method, by the application of the blast treatment to the core wire and the surrounding wires formed with plated coatings, the adhesive property of the resin coating with respect to the plated coatings of the core wire and the surrounding wires is improved, and the rustproof performance of the resin coatings is improved.
In the first to third prior arts described above, the rustproof coating is formed by temporarily untwisting and spreading the twisted portion of the PC strand in sequence, feeding the same in sequence while keeping the spread state, causing the synthetic resin powder coating material to be adhered to the entire outer peripheral surfaces of the core wire and the surrounding wires, heating and melting the adhered coating material, and forming the synthetic resin coating as a rustproof film. However, there is a risk of damage being formed to the surface of the film such as partial peel-off or scratch of the synthetic resin coating due to reception of an external force during transport, unloading or insertion of cable into a sheath at the time of construction. There is a problem in that the steel wire in the interior may be eroded if water drops containing salt enters from the partial surface damage portion or a pinhole when the PC strand having such surface damage generated thereon is used as a cable for a material to be placed in a tense state or a cable-stay material for marine structures or cable-stayed bridges.
In the fourth prior art described above, usage of the PC strand in which the core wire and the surrounding wires are untwisted and plated is disclosed. However, the adequate thickness of the plated core wire and surrounding wires, that is, adequate diameters of the respective wires are not figured out at all. Therefore, twisting pitches of the surrounding wires with respect to the core wire may become short or long, that is, uneven, and there may arise a case where a part of the twisted surrounding wire is twisted without coming into contact with the core wire (state of being separated therefrom). In any cases, there is a problem in that when a predetermined tensile strength is applied thereto at the time of usage as the PC strand, a tensioning force caused by the tensile force is intensively applied to a part of the core wire or the surrounding wires, so that the corresponding part may be expanded or broken, and hence the tensile strength equivalent to a bear PC strand having no coating cannot be obtained.
By the way, a plated layer of a steel material, for example, galvanization is a rustproof means having two effects; namely a coating action and a sacrificial anode action, and is a depleting material which is gradually depleted when exposed in the atmosphere. Since the coating of the galvanization is bound with oxygen, the layer has a high density, so that a high rustproof effect is expected by coating the surface thereof or the like. In addition, although the galvanization itself goes rusted (gradually dissolved) by contact with moisture as an object of rusting, the steel material is protected by its sacrificial anode action. In other words, it is a self-sacrificial anticorrosive effect that is dissolved self-sacrificially and prevents generation of red rust of the steel material. Even if a part of the plated layer has got damaged, the damaged portion is protected by the sacrificial anode action of the plated coating therearound. Therefore, formation of rust on damaged portion as in the case of coating does not occur. Being superior in bendability, the flexibility as a characteristic of the PC strand and the stability of fixing performance are secured, and hence the galvanization is used often as the rustproof material for the PC strand.
However, since the galvanization is depleted, a permanent effect is not expected. Although the problem does not occur in ten to twenty years in a normal environment, rust may be formed in approximately two to three years in an undesirable environment such as marine or coast. The thicker the galvanization layer, the more the rustproof becomes effective. However, since the surrounding wires are twisted around the core wire in the PC strand, if a thick plated layer is formed on the outer periphery of the wires of the PC strand, a thickness six times as much as the plated layer affects on the outer diameter of the PC strand, which is not up to the standard. Therefore, the thickness of the plated layer cannot be increased more than is necessary. Furthermore, the twisting pitches of the core wire and the surrounding wires may become short or long, that is, uneven, because the wires cannot be twisted with regular pitches unless the diameter of the core wire is set to be slightly larger than the diameter of the surrounding wires. Consequently, there arises a problem that an intensive tensile force is applied to the core wire or a part of the surrounding wires and hence the wires are partially expanded or broken, that is, the general strength thereof as the PC strand is lowered.
Therefore, in the PC strand of the prior art, it is an object to improve and stabilize the tensile strength as the PC strand to allow a long term use by preventing corrosion due to the entry of water drops from the partial surface damage portion of the rustproof coating or from a pinhole, or by preventing the winding pitches of the surrounding wires from becoming uneven by setting the diameters of the core wire and the surrounding wires respectively so as to make the winding pitch of the surrounding wires with respect to the core wire constant.